The present invention relates to a means for regulating the transfer of heat to and from the surface of a container. More particularly, the present invention relates to a temperature regulating means adapted to mount on the lower surface of a liquid container, such as a coffee pot, for maintaining the temperature of the container within a suitable range regardless whether liquid is contained therein, said temperature regulating means utilizing a moving blanket of air relative to the lower surface of said container in order to regulate its temperature.
Containers which are used for containing coffee and other liquids during heating and temperature maintenance of the liquid must be able to withstand the thermal abuse to which they will be subjected. Such containers are typically made of glass and are heated by heating elements which provide a somewhat constant rate of heat generation without regard to the temperature of the container. In fact, virtually all commercially available heating plates intended for use with coffee pots have 100 Watt heating elements which have no temperature controls other than their main power switch. This arrangement may be relatively effective in common circumstances in which the container is periodically refilled with liquid since heat from the heating element is transferred to the container and subsequently conducted to the liquid. The liquid not only enables dissipation of the heat but even slightly refrigerates the container since the liquid is continuously subject to cooling due to evaporation at its upper surface.
However, such a heating arrangement is much less effective, and in fact becomes quite hazardous, when there is little or no liquid contained within the heated container. Such a situation results in a variety of instances and commonly occurs due to complete evaporation of the liquid within the container. Heat can only be conveyed from an empty container through means of convection or radiation of the heat between the container and the gaseous air that surrounds it, processes which are slower and less efficient than the mechanisms acting when the container is filled with liquid. The "hot spots" of standard heating elements are known to reach 450.degree. F. Therefore, when a container is left empty on an activated heating element, its temperature tends to become excessive as well.
Furthermore, even when the temperatures do not reach extremely critical levels, the material of a coffee container is subject to harsh thermal changes. In most circumstances, a coffee container is continually being removed from its burner in order to pour a cup (allowing the container to cool) and then replaced back onto its heating element which has been increasing in temperature while the container was removed. Commonplace events like that subject a coffee container to harsh thermal changes which create internal stresses in its material.
As a result of excessive or rapidly changing temperatures and the internal stresses caused thereby, thermal cracking of the container may occur. Such cracking may, often without notice, cause the ultimate failure of a container which, especially with a hot pot of coffee, can lead to scalding and other injuries. Such cracking is especially likely when the container accidentally or otherwise strikes against another object or when it is suddenly quenched cooled such as when subjected to cold water.
Furthermore, it is particularly desirable to replace the conventional glass material of coffee containers with materials that are less costly, less likely to break, and more lightweight. Plastic materials present such benefits but their use is encumbered by a variety of other problems. To begin with, many plastics do not meet federal standards required for containing coffee. Many plastics are also vulnerable to the high temperatures encountered on conventional heating elements designed for heating coffee. In fact, of the plastics known which do meet the federal and industry standards, there is none which can withstand the temperatures that may be encountered on conventional heating elements. Not only are plastics more susceptible to melting than glass, but enduring high temperatures also tend to shorten its life otherwise. Plastics are subject to several failure mechanisms similar to those previously discussed in addition to softening and melting, and they may also become discolored due to oxidation or concentrated dislocations caused by thermal stress.
Many attempts have been made to provide plastic containers without much success. One such attempt is to bond epoxy or other high temperature materials directly to the bottom of a plastic carafe. As soon as the bonded material reaches the temperature of the warming plate, the plastic carafe deteriorates rapidly from causes such as blistering, cracking, heat distortion, heat crazing, etc. Short term exposure to a heating plate is alright for most conceivable plastic containers as long as the carafe is full of liquid, but after long use, even when filled with liquid, containers often fail. Worse, though, when a heated container is empty due to evaporation or other causes, it tends to fail more prematurely.
Other attempts have resulted in designs which tend to unnecessarily limit contact with a heating plate due to the shape of the carafe's bottom. Another attempt was to fabricate a carafe in two pieces, with the bottom section metal and the upper section being plastic, resulting in a seam on the inside of the carafe. Unfortunately, such seams are considered unsanitary and substantial amounts of heat are concentrated at the interfact between metal and plastic, therefore causing premature failure.
It is therefore an object of the present invention to provide a temperature regulating element which regulates the transfer of heat to and from a container for containing liquid. It is also an object of the present invention to provide a means for heating a container to a desired temperature in a regulated manner by utilizing heat from a source of continuous heat production without damage to the container.
Another object of the present invention is to provide a means for maintaining the flavor of coffee warmed in a container on a standard heating element.
It is a further object of the invention to minimize the possibilities of stress cracking in the material of a container heated on a standard heating element.
Further, it is an object of the present invention to provide a temperature regulating means which is adapted to be mounted on the lower surface of a standard container for containing coffee or other liquids; and it is yet another object of the present invention to provide a container for containing heated liquids, which container is permanently secured to a means for insulating the base of said container.